The subject matter described herein relates generally to methods and systems for wind turbines, and more particularly, to methods and systems for protecting a wind turbine and a wind farm against inrush currents after grid recovery.
Generally, a wind turbine includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower.
Some wind turbine configurations include double-fed induction generators (DFIGs). Such configurations may also include power converters that are used to convert a frequency of generated electric power to a frequency substantially similar to a utility grid frequency. Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Alternatively, some wind turbine configurations include, but are not limited to, alternative types of induction generators, permanent magnet (PM) synchronous generators and electrically-excited synchronous generators and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator.
Known wind turbines have a plurality of mechanical and electrical components. Each electrical and/or mechanical component may have independent or different operating limitations, such as current, voltage, power, and/or temperature limits, than other components. Moreover, known wind turbines typically are designed and/or assembled with predefined rated power limits. To operate within such rated power limits, the electrical and/or mechanical components may be operated with large margins for the operating limitations. Such operation may result in inefficient wind turbine operation, and a power generation capability of the wind turbine may be underutilized.
Nearly every power generating plant—be it a nuclear power plant, a cogeneration plant, a hydro power station, coal power station, a gas power station, or a wind power station, in the following also referred to as wind farm—generates the current by one or more generators which converts mechanical energy into electrical power. Typically, the electric power of the generators is feed into the utility grid via transformers. For example, the generators of the wind turbines of a wind farm are typically connected to individual transformers coupled via a main transformer of a substation to the utility grid. When the utility grid comes back after an outage, the individual transformers of the wind turbines are still connected and are all energized at the same moment. This may result in a very high inrush current peak. Accordingly, the main transformer in the substation may be disconnected from the utility grid due to an unwanted protection relay tripping. Furthermore, the inrush current peak may lead to resonances and/or oscillations in the power distribution system and/or to high mechanical loading of the transformer windings which may result in reduced transformer lifetime.
In view of the above, there is a desire for improved inrush current reduction and/or inrush current protection of power generating plants, in particular wind farms.